1. Field of the Invention
The invention is in the field of imaging devices and more particularly in the field of portable high-resolution three-dimensional ultrasonic imaging.
2. Description of Prior Art
Ultrasonic imaging is a frequently used method of analysis. The technique is used to examine a wide range of materials and is especially common in medicine because of its relatively non-invasive nature, low cost, and fast response times. Typically, ultrasonic imaging is accomplished by generating and directing ultrasonic waves into a material under investigation. This ultrasonic imaging uses a set of ultrasound generating transducers and then observing reflections generated at the boundaries of dissimilar materials, such as tissues within a patient, also uses a set of ultrasound receiving transducers. The receiving and generating transducers may be arranged in arrays and are typically different sets of transducers but may differ only in the circuitry to which they are connected. The reflections are converted to electrical signals by the receiving transducers and then processed, using techniques known in the art, to determine the locations of echo sources. The resulting data is displayed using a display device, such as a monitor.
The beam intensity as a function of position may oscillate rather than fall off monotonically as a function of distance from the center of the beam. These oscillations in beam intensity are often called “side lobes.” In the prior art, the term “apodisation” refers to the process of affecting the distribution of beam intensity to reduce side lobes. However, in the remainder of this specification the term “apodisation” is used to refer to tailoring the distribution of beam intensity for a desired beam characteristic such as having a Guassian or sinc function distribution of beam intensity (without the side lobes).
Steering refers to changing the direction of a beam. Aperture refers to the size of the transducer or group of transducers being used to transmit or receive an acoustic beam.
The prior art process of producing, receiving, and analyzing an ultrasonic beam is called beam forming. The production of ultrasonic beams optionally includes apodisation, steering, focusing, and aperture. Using a prior art data analysis technique each ultrasonic beam is used to generate a one dimensional set of echolocation data. In a typical implementation, a plurality of ultrasonic beams are used to scan a multi-dimensional volume.
Typically, the ultrasonic signal transmitted into the material under investigation is generated by applying continuous or pulsed electronic signals to a transducer. The transmitted ultrasound is commonly in the range of 40 kHz to 10 MHz. The ultrasonic beam propagates through the material under investigation and reflects off of structures such as boundaries between adjacent tissue layers. As it travels, the ultrasonic energy may be scattered, resonated, attenuated, reflected, or transmitted. A portion of the reflected signals are returned to the transducers and detected as echoes. The detecting transducers convert the echo signals to electronic signals for processing using simple filters and signal averagers. After beam forming, an image scan converter uses the calculated positional information to generate two dimensional data that can be presented as an image. In prior art systems the image formation rate (the frame rate) is limited by at least the return time of an ultrasonic pulse. The pulse return time is the time between the transmission of ultrasound into the media of interest and the detection of the last reflected signals.
As an ultrasonic pulse propagates through a material of interest, additional harmonic frequency components are generated, which are analyzed and associated with the visualization of boundaries, or image contrast agents designed to re-radiate ultrasound at specific harmonic frequencies. Unwanted reflections within the ultrasound device can cause noise and the appearance of artifacts in the image.
One-dimensional acoustic arrays have a depth of focus that is usually determined by a nonadjustable passive acoustic focusing means that is affixed to each transducer. This type of focusing necessitates using different transducers for different applications with different depths of focus.
Two-dimensional transducer arrays used for high-speed three-dimensional imaging applications suffer from sensitivity loss caused by coupling multiple signal transfer and distribution systems to ultrasound systems. Two-dimensional transducers used for high-speed three-dimensional imaging applications must have a large number of pixels for two-dimensional steering capability with high resolution. High numbers of radiating/receiving pixels inevitably result in high electrical impedances per pixel in many types of transducers (e.g., piezoelectric, capacitive Micro ElectroMechanical (MEM) transducers), making high-resolution two-dimensional arrays impractical.
To reduce the impedance, many prior art devices use a limited number of elements, or a one-dimensional array. In typical ultrasound systems, these high impedance elements are driven by a typical coaxial cable bundle carrying as many micro-coaxial cables as the number of pixels, with each micro-coaxial cable usually having 50-75 Ohm impedance. These cables do not directly interconnect to the individual elements of the two-dimensional array. Another level of interconnection in the form of multi-layer Printed Circuit Boards (PCBs) co-fired ceramic boards or multi-layer flexes must transfer the signal to the transducer elements. The transducer elements are grouped into pixels each containing one or more transducer elements. For example, each pixel may contain one transmitting and one receiving transducer element. Systems including cables suffer from drawbacks that include, (1) the large number of required micro-coaxial elements makes the cable bundle unwieldy, and (2) the 50-75 ohm cable impedance cannot efficiently interface with or match the high electrical impedances of the individual transducer elements. These drawbacks result in impractically low sensitivity levels. The use of an additional multilayer transition device to connect from cables to transducer elements, introduces additional capacitive loading and crosstalk.
FIG. 1 shows a prior art ultrasonic imaging device 100, including a system 102, a first connector 104, a cable 106, a second connector 108, and a hand held unit 109, which includes a multilayer structure 110 for transmitting the signal, interposing electrical connector structure (not shown), acoustic elements 114, and pins 116. System 102 may include a systems motherboard. The multilayer structure 110 could be a PCB, co-fired ceramic board, or flex circuits, for example. The interposing electrical connector structure (not shown) could be an interposing media for carrying signals from the multilayer structure 110 to acoustic elements 114. The device of FIG. 1 is bulky and can be difficult to move and manipulate because cable 106 has many wires in it and therefore interferes with movement. Cable 106 also needs to be thick and therefore does not bend easily. Handheld unit 109 has electrical contact pads 116 for individually powering the acoustic elements 114.